An early start for the Panama land bridge

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Science  10 Apr 2015:
Vol. 348, Issue 6231, pp. 186-187
DOI: 10.1126/science.aab0099

The birth of the Panama land bridge, which connects the Americas, has been associated with one of the biggest biological exchanges in Earth history as numerous species migrated from one continent to the other (1). Nevertheless, the timing of formation of the land bridge is still much debated (2). On page 226 of this issue, Montes et al. (3) propose that the Central American Seaway, which separated South and North America, closed about 15 to 13 million years ago, more than 10 million years earlier than previously thought (4), with important implications for ocean circulation, climate, and biotic exchange.

The authors studied ancient river deposits found in northern Colombia. Through a combination of geochronological analysis and geological mapping, they determined the age, origin, and pathway of this river. They show that the river system began to flow about 15 to 13 million years ago and could not have originated in any other place than the volcanic arc of Panama. This means that a terrestrial connection must have existed between northern Colombia and the river's source area at this time. A marine connection, the authors state, could only have consisted of transient shallow channels west of today's Panama Canal (see the figure). With this, they contest alternative scenarios (4), which suggest that a deep-sea strait separated the Americas until 3 million years ago, preventing massive biotic exchange before this time.

Montes and colleagues first questioned the generally accepted young age of the seaway closure in 2012 (5). Before this, however, some geologists and biologists had already suspected that the land bridge might have emerged earlier than presumed. Paleontologists found that an array of “herald animals” such as camels, peccaries, horses, beardogs, and rhinoceroses were at the bridge by 15 million years ago and that giant sloths, terror birds, and plants (among others) passed the bridge in the northern direction well before 3 million years ago (6). Bacon et al. have also inferred early exchange of plants between the continents from dated molecular phylogenies (7).

Panama then and now.

According to Montes et al. (3), a Panama land bridge with an “Inter-American” river system formed during the Miocene (15 to 10 million years ago). Support for this land bridge comes from fossils of herald animals that crossed the land bridge in either direction well before 3 million years ago (6). (Inset) Map of Central America today.

Various scenarios were invoked to explain these observations, but none of them were conclusive. A crucial point of controversy has been whether early biotic exchange reflects a terrestrial connection or was a result of random long-distance dispersals (7) or perhaps climatic cooling (8). With excavations well on their way for renewal of the Panama Canal, a wealth of new paleontological data (2) and comparative molecular analyses (7) is enforcing the idea of an early land bridge.

Other evidence for an early emergence of the land bridge comes from the magmatic rocks in Panama and northern South America. Changes in the geochronological composition of the Panama volcanic arc at about 24 million years ago mark its collision with the South American island continent (9). This collision represented the end of a 2- to 3-thousand-km oceanic voyage of the Caribbean plate on which embryonic Panama was situated (10) and formed the prelude to complex deformation at the triple junction of the Caribbean, South American, and Nazca plates. In essence, subduction processes built the fundaments of Panama and ultimately determined the pace of its emergence (10).

The formation of the Panama land bridge was not an isolated event. As the Caribbean plate scraped along South America, it reconfigured the northern South American landscape (9), resulting in the characteristic trifurcate shape of the northern Andes. Shoaling of the Central American Seaway also coincided with large changes in the South American lowlands corresponding to today's Amazonia, where a mainly fluvial landscape changed into a vast wetland that developed as a consequence of deep subsurface processes, the rising Andes, and related climate change (11). Wetland formation, contemporary marine influence in Amazonia, and an extended inland coastline (12) are all related to the Caribbean collision and closure of the Central American Seaway.

Pushing back the age of the Panama land bridge by more than 10 million years is noteworthy. However, an early terrestrial connection reconciles an array of seemingly odd results, such as the early dispersal of freshwater fishes between South and Central America (13) and shoaling, changes in oceanic currents, and deep-water exchange between the Pacific and Atlantic between 12 and 7 million years ago (6, 14, 15). These events are difficult to explain if the Central American Seaway did not close until about 3 million years ago.

The geological evidence presented by Montes et al. lends support to the idea that the Great American Biological Interchange (GABI) between the Americas started millions of years earlier than commonly assumed. But why did many organisms wait before migrating around 3 million years ago? Molnar (8) has suggested that the peak in biological exchange about 3 million years ago was a result of climatic cooling and the formation of savannas suitable for herbivore migration. A full understanding of the dynamics of the GABI will require better knowledge of the early land bridge and its environments. Data are also needed on the existence and duration of any intermittent transoceanic connections elsewhere along the narrow strip of land that separates the Atlantic and the Pacific.

References and Notes

  1. Acknowledgments: We thank D. van Hinsbergen and A. Antonelli for discussions and valuable comments.
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